Electromechanical systems used in high-accuracy positioning applications which use hydraulically and pneumatically transmitted force are known; pneumatically-controlled systems used in material handling and other such similar operations are also known. Until recently, pneumatic systems were almost entirely limited to on/off control, however, the need for increased capabilities in such systems has lead to the development of programmable pneumatic equipment capable of incremental control to produce intermediate positions along a path.
Such prior control systems operate in either an open or closed-loop mode. In a closed-loop system, an output position is continuously monitored and compared to a set point (the desired point), and the system strives to reduce any deviation of position from the set point to zero. In an open-loop system, output positions are obtained by proportioned signals, which are assumed to provide a current position, and any deviation between the actual position and the set point is ignored.
Pneumatically driven systems contain many unpredictable and time dependent variables that prevent open-loop control systems from providing reliable positioning of a load and that interfere with the effective use of closed-loop control systems in most applications. Such variables include the varying break-away forces needed to move sealed pneumatically driven elements, such as pistons and valves, because of the age and condition of the seal-forming members and the condition of the interfacing surfaces against which the seal-forming members form their seals. Such variables also include the varying frictional forces imposed on moving seal-forming members because of their age and condition and because of the condition of the interfacing surfaces over which such seal-forming members are driven. This complex and unpredictable set of variables is further complicated by variations of load and vertical forces due to the age and conditions of the load and the movable joints interconnecting the load and the pneumatic load driving members, the varying positions of the load with respect to earth and the varying contributions of the force of gravity to the load forces imposed on the pneumatic load driving members because of the varied positions and desired changes of position of the load and, of course, due to the break-away and moving frictional forces of the moving elements of the load.
Furthermore, the reliability and effective operation of closed-loop pneumatically driven positioning systems is comprised by variation in the effective force imposed on pneumatically driven members because of unpredictable variation in the applied pressure, pressure build-up and pressure application times due to the variable factors described above, limitations in the ability of pneumatic pressure controlling devices to maintain constant pressures within close tolerances and their wear and increasing unreliability due to age, the conditions of the orifices, passageways and elements through which operating air pressures are applied to pneumatic load driving members. It must be remembered, of course, that compressed air generally includes and carries water vapor that is not always reliably removed from the compressed air by air dryers and this water vapor imposes another unpredictable and time dependent variable on pneumatically driven positioning systems. Finally, the human operators themselves aggravate an already enormously complex and unpredictable problem with their unpredictable behavior.
The ability to provide reliable closed-loop pneumatic positioning systems is even further complicated by the inadequacies of available pneumatic components.
Closed-loop pneumatic control systems contain several components generally including a positioning drive, i.e., a controlled valve and a pneumatic cylinder, a load as the driven element, a feedback sensor or displacement pickup, an electronic controller, and an electronic amplifier that adjusts the controller power level to drive the controlled valve.
The positioning drive commonly includes a continuous-acting valve as the control element. The valve is a vital element of the control loop. The precision and speed of the valve must exceed that of the control loop as a whole. Thus, requirements for the control loop generally define the valve choice. Examples of continuous-acting valves available for pneumatic applications include digital, proportional, and servo valves. Digital valves use an electrical step motor to position the valve spool. An important disadvantage of digital valves is their lack of response due to step motor speed limitations.
Proportional valves use proportional solenoids for direct actuation of a valve spool. If spool travel is not controlled by any other means, these valves suffer from hysteresis, and in either case are not very fast.
Servovalves, on the other hand, are extremely fast and precise, however, they are expensive and their cost limits their uses.
Generally, the response of the control element must be faster than that of the control loop as a whole. Valve speed determines how rapidly the system compensates for unwanted deviations. One rule of thumb is that the stability of a closed-loop controlled system requires the valve natural frequency to be at least three times that of the cylinder and load to be moved. For most material-handling operations, the cylinder natural frequency alone is in the 1 to 5-Hz range.
Most valves, with the exception of digital valves, are analog in operation. Hence, the electronic amplifier is usually analog to correspond to the controlled valve. The amplifier drives a solenoid output stage, for example, by controlling valve-spool travel.
The controller may either be analog or digital. Both types require set point inputs to provide specified outputs such as required cylinder position. Accuracy of the output variable depends on the consistency of the set point input. Noise and drift problems are inherent to analog controllers, and can have a detrimental effect on system position. Digital controllers do not experience these problems and are preferred. Because of all of the above variables, it has been virtually impossible for pneumatic positioning systems to make reliably small changes in local position, for example, because of unpredictable and variable break-away forces and inertial forces, and to achieve reliable position for these reasons and all of those set forth above.
As noted above, pneumatic closed-loop positioning systems are substantially affected by fluid compressibility among other factors. The result is a complex, completely non-linear closed-loop system, where variations in constants cannot be reliably corrected for wear and age, stroke, pressure, load and the like.